It is well known to apply a coating, called a ground coat or primer coat, to metallic or non-metallic substrates which are sensitive to corrosion or to premature aging, in order to protect them against environmental factors. Examples of substrates requiring protection include, for example, hulls, interior and superstructures of ships, off-shore oil drilling platforms, metallic frames, bridges, automobile bodies and related equipment, storage tanks, guardrails, and aircraft equipment.
All these substrates, when insufficiently protected, undergo chemical transformation actions, initially superficial and then more in depth, reducing their physical and particularly their mechanical characteristics, when they are exposed to severe natural or synthetic environmental conditions, such as heat, cold, ultraviolet radiation, moisture, particularly saline, wind, rain, sea water, snow, particle impacts and other harmful factors due to atmospheric pollution or to liquid or gaseous fluids resulting from the chemical, biochemical, biological and other industries.
Heretofore, depending on the substrate and its environment, there have been used as protective coatings various compositions prepared in aqueous medium or in organic solvents containing polymers as diverse as, for example, phenolic resins, chlorinated rubbers, polyesters, polyurethanes, epoxy resins, also containing reactive pigments such as zinc, lead, barium, calcium and strontium chromates, and zinc phosphates, or metallic oxides such as iron, titanium and zinc oxides, and also containing zinc in fine particulate form.
More particularly, the prior art describes and recommends anti-corrosive coatings containing reactive pigments based on epoxy, phenoxy, and/or vinyl chloride resins, in aqueous medium or organic solvents, for the protection of metallic substrates, making use of the reactivity of the epoxy or phenoxy groups with curing agents such as aliphatic or aromatic polyamines, aliphatic or aromatic polyamides, melamine, polyisocyanates, or urea-formaldehyde resin, thus resulting in homogeneous systems by an addition reaction.
Serious toxological problems arising from utilization of the various chromium salts of zinc, lead, barium, calcium and cobalt have greatly reduced the use thereof in paint formulations. Industrial and governmental restrictions at all levels further limit the use of these effective corrosion resistant pigments, thus rendering it more difficult to formulate effective corrosion resistant coatings.
Since development in the late 1930's zinc-rich primers have been considered optimum anti-corrosion coatings on iron or steel substrates. However, numerous problems have restricted their use generally to maintenance type rather than industrial type primers. This limitation is due to the nature of the anti-corrosion reaction of the zinc dust. As explained in Schering Industrial Chemicals: Surface Preparation I, page 29:
"Zinc powder in rust inhibiting paints has a different mode of action than the usual rust inhibiting pigments. The action is based on an electrochemical interaction between the zinc powder and the steel substrate it is meant to protect. In order not to insulate the various pigment particles from each other it is therefore necessary to work with very little binder. A satisfactory corrosion protection is only achieved when the pigment:binder ratio is at least 92:8 (optimum 95:5). The maximum recommended film thickness is about 50 um."
This high zinc to binder ratio required to provide effective cathodic protection has been accepted by the entire coating industry.
The high zinc level and the relatively high density of zinc powder often cause undesirable settling during short term storage. Hence, the zinc powder is often added just prior to application and mixed rapidly during application to prevent settling and clogging of spray equipment. This is a deterrent to efficient field use.
Single component soft settling zinc dust formulations do not utilize binders with low water vapor transmission properties, thereby greatly reducing the optimum qualities which could be provided by coreacted hydrophobic systems such as epoxy, phenoxy and vinyl polymers.
A somewhat lower zinc content is disclosed in U.S. Pat. No. 3,998,771, issued December, 1976 to T. J. Feneis, Jr. et al, which describes water-based coating compositions for application on iron supports to obtain anticorrosive coatings which include in a single phase composition:
(a) about 2% to 10% by weight of a non-volatile, liquid epoxy resin, with low viscosity, derived from bisphenol A and an epihalohydrin, for example, epichlorohydrin; PA1 (b) about 2% to 10% by weight of a modified polyamide, i.e., an addition product of a water soluble polyamide and a liquid epoxy resin; and PA1 (c) about 55 to 70% by weight of a zinc-powder pigment having an average particle size of about 2-15 microns.
U.S. Pat. No. 4,417,007, issued November 1983 to G. A. Salensky et al, discloses a one phase composition containing from about 4% to 25% by weight epoxy or phenoxy resin binder and polyamine hardener, about 43% to 90% by weight zinc dust, about 3% to 38% by weight Mn.sub.3 O.sub.4 fume pigment, up to 35% by weight additional pigments including pigment extenders and fillers (such as talc, clays, diatomaceous silica and silica), up to 5% by weight pigment suspension agent (such as hydrous magnesium silicate and lecithin), and balance organic solvents. A 1:1 volume ratio of zinc dust:Mn.sub.3 O.sub.4 is preferred.
However, although these epoxy resin-based coatings for metallic or non-metallic substrates have very appreciably improved the useful life of the substrates, it has been found that the best coatings proposed by the prior art still exhibit major disadvantages which are manifested, for example, by blisters, separation from the substrate to be protected, evidence of poor adhesion and/or of premature corrosion, thus requiring frequent restoration of the protective coating.
Anti-corrosive coating compositions of the type having a very high zinc content are described in the book ZINC DUST AND POWDER: THEIR PRODUCTION, PROPERTIES AND APPLICATIONS, C. Bradford, Hafford et al, First Edition, May 1982, published by International Lead Zinc Research Organization, Inc., 292 Madison Avenue, N.Y. 10017 pages 90-91. Such anti-corrosive coating compositions with very high contents of zinc powder have the reputation of permitting production of protective films equivalent to an anodic metallic coating. In order to accomplish this, these compositions contain from about 80 to 95% by weight, based on the total weight, of zinc powder, less than 1.8% by weight of a pigment suspension agent (pyrogenous silica) and accordingly, a very low content of binder, i.e. of film-forming polymers. This low binder content is considered an advantage by those skilled in the art, since an excess would, in the dried coating, isolate the metallic grains from each other and also from the substrate to be protected. It is considered that contact of the metallic grains with the substrate must be perfect, according to Volume II: Peinteures et Vernis (Paints and Varnishes) by P. Grandou and P. Pastour--1982 Edition, published by Hermann, Paris, page 39.
Performance of Zinc Dust in an Epoxy Polyamide Coating, by T. K. Jones, published by Matthiessen & Hegler Zinc Co. (publication date unknown) summarizes tests performed on coatings containing from about 70% to 95% by weight powdered zinc in an epoxy resin-polyamide binder. The binder system was the conventional two-component type wherein part A contained zinc dust, epoxy resin, polyamide resin, minor amounts of bentonite, urea-formaldehyde resin and silica gel, and solvents; and part B contained a polyamide hardening agent and solvents. Optimum results were obtained with 0% to 95% zinc dust.
However, the use of a high concentration of metallic pigments results in disadvantages in addition to those described above, which are exhibited during the preparation, storage and/or application of the anti-corrosion coating compositions.
One disadvantage results from the high reactivity of the metallic pigments with respect to certain substances present in the anti-corrosive coating compositions. This reactivity results in a significant development of the rheological characteristics of the compositions, for example a rapid and troublesome increase in viscosity with time and the appearance of a gel or even caking of the compositions before application.
A further disadvantage lies in the fact that it is extremely difficult to have available an anti-corrosive coating composition which simultaneously has a high concentration of reactive metallic pigments and sufficient binder to permit easy handling or optimum surface adhesion. These two requirements are diametrically opposed to each other with the result that it is difficult to deposit compositions with a high concentration of metallic pigments on the substrate to be protected, and therefore protective coatings with only 5% to 10% binder are produced with poor adhesion characteristics. Blasting or other pretreatment of the metal substrate is thus necessary in order to secure adequate adhesion.
Additionally, the surface of an anti-corrosive coating with a very high zinc content, after application to the substrate to be protected, remains relatively rough and even irregular in appearance and therefore does not permit the deposition of a sufficiently aesthetic top coat for many industrial applications, such as the bodies of automobiles, trucks, buses and other industrial equipment which require a very smooth and glossy surface for exposed parts.
Another disadvantage also arises from the fact that the substrate to be protected, particularly if of iron or steel, must undergo preparatory surface treatment by at least one mechanical process (such as sanding, brushing, shot blasting, hammering or burn-off), and/or a chemical process (such as descaling with an acid or pickling treatment).
Finally, another disadvantage resides in the fact that prior art organic anti-corrosive coating compositions have a capacity for protecting metallic substrates against corrosion which is limited in performance with depositions of .+-.3 mils (75 microns) to an average of 1500 hours in salt spray by the ASTM B117 standard. It is highly desirable to improve this performance especially for metal substrates exposed to particularly aggressive media.
It is an object of the present invention to provide improved low cost coating compositions for the protection of metallic or non-metallic substrates which avoid or reduce the disadvantages of prior art compositions outlined above.
A further object of the invention is the provision of a zinc-based coating composition which can be easily applied over untreated metal substrates and used in all conventional application equipment while at the same time meeting all current governmental environmental regulations regarding solvent emissions for volatile organic compounds (VOC).
A still further object is to provide an anti-corrosive coating composition which exhibits an improved capability for deposition on a substrate to be protected; which provides improved adhesion between the coating and substrate; which is flexible, resistant to chemical agents and mechanical and thermal shocks; which exhibits an excellent dimensional stability and low hardening shrinkage; and which provides improved protection life as compared with the best anti-corrosive coatings currently available.